Exposure Notification Express is meant to make it easier for states and their public health authorities to use the Exposure Notification system without building or maintaining a full mobile app. The system is handled within the Settings app within iOS and Android devices.
This could make it easier for users to opt in to the exposure notification system and increase usage of it. If you're in a state that supports the exposure notification system, you'll receive a notification letting you know that that system is available. Tapping it will open up your Settings app where you can turn on exposure notifications without an app (via MacRumors).
Virginia, Washington DC, Nevada, and Maryland will be among the first places to deploy Exposure Notification Express, with more states expected to add it later this year.
As with the standard exposure notifications, the new Exposure Notification Express uses Bluetooth to determine if you're near another person. If that person confirms with their device that they've tested positive for COVID-19, you'll be notified if you were near enough to them for a possible exposure.
In addition to rolling out to Apple devices with the release of iOS 13.7 today, the new Exposure Notification Express system will also be available on devices running Android 6.0 or higher beginning later this month (via The Verge).